clevtool
Well Known Member
This thread was started inside a different thread so I am moving it here to make more sense...
Russ,
Sorry my reply has taken so long. I had made up the page that will be linked below, but didn't want to switch this to a dimple die thread. Since there is a dimple die thread going on, I decided that I would post there. I do still intend on getting a tutorial on how to 'see' the difference in results of dies and techniques.
<snip from: http://www.vansairforce.com/community/showthread.php?t=81949&page=2>
At Cleaveland we perfected the 1942 springback dimple die design. The term springback refers to the angle of the 'faces' of the dies rather than the angle of the 'funnel'. The male die has a concave face of a couple degrees where the female has a convex face. These angles as well as some of the other diameters and angles are critical as to how they fit the rivet, and how flat the material ends up after dimpling. Most of our competitors now have attempted to copy our 1992 die geometry, some with more success than others, but they still don't measure up to what a Cleaveland Dimple Die can do. To read more about exactly how our dies were perfected and how we keep them that way you can read the following page that I have put together to answer this question.
http://www.cleavelandtoolstore.com/custompages/springback-dimple-dies.html
As some builders don't know what to look for I will create a short tutorial on how to evaluate the dimpled skin and post it here shortly.
The statement about the tank dies for the substructure is certainly correct. The tank dies are about .007" deeper than standard to account for proseal under the rivet head. They work better for nesting than two dimples from the same die. Most builders don't want to buy another set for the substructure, but for those discerning builders we do carry a "substructure" die (DIE4263SS) that is .011" deeper so that the dimples will nest perfectly.
-
Mike Lauritsen - Cleaveland Aircraft Tool
Nice question Russ! After 20 years I don't hear new questions very often unless it is in reference to a new kit or new tool...
First off the cylindrical hole is important, that is why we have been suggesting #40 and #30 reamers to enlarge the prepunched holes since the first prepunched kits. The reamer with it's 45deg. entry bevel was designed to start itself perfectly centered in a hole and enlarge it to a perfectly sized and perfectly cylindrical hole. Machinists have been doing this 'forever' and many other industries know that the only way to get a good fit and finish is to ream a hole. With the matched hole prepunch the reamer takes the same time as a drill with better results... why wouldn't you ream them? There is less burr because with the drill bit 'chattering' because of the lack of center to follow causes 'tearout' and triangular shaped holes. This tearout makes deburring a challenge and often leads to over deburring. Over deburring by just 0.0025" on each side of a 0.025" skin takes 20% of your skin thickness away!! But all that is old news (question answered many times before).
The interesting part is the tapered pilot die. We decided against that idea right away, and here is why. If you are using a good quality tool to align the dies (like the C-frame or squeezer) the male and female die will be lined up with each other. (This is why a squeezer that has the ram going through the yoke is essential, when the yoke has pressure applied, it will rock back, the ram MUST go with it or alignment is not only lost, but due to the side load now induced the whole process gets out of control). If alignment is maintained, the male will be concentric with the female, the straight shank die will slip through the hole and the funnel of the die will center the cylindrical hole on the cylindrical die funnel. This lines the hole up with the die. If the male pilot was slightly tapered the side load forces on the sheet would climb quickly and we were afraid of what that might mean for the hardness of the Aluminum at that critical point for cracking. As the dimple is made the hole expands anyway so any tight tolerance in the pilot/hole interface is lost. We did do extensive testing to look at the behavior of the Aluminum sheet of various thickness under a microscope when smaller diameter holes were used with a smaller diameter pilot ( pay attention #41 guys
). Our goal was to make a die and hole combination (this is prior to prepunched kits) that would result in a tight fit with a 3/32" rivet. Radial stress cracks occurred quickly when reducing the pilot size. When we started with screw dies we again did similar testing so that standard clekos could be used for #6 and #8 screw holes. Again we couldn't get it done because of the stress cracks induced. We finally determined that with millions of hours behind the tried and true #40, #30 starting hole, safety is on the side of keeping it there. After learning about strength of materials in engineering classes I am quite happy that we did not find a way to make the un-expanded 3/32" rivet work. I have never done the math but I am guessing that is a fairly significant reduction in area.
As for the last question about the sides of the cylindrical hole being vertical... I have never heard reference to that and doubt it's origins. I can see how that could be done, but I don't see much advantage other than the shop head having a bit flatter surface to sit against. This would also make the rivet need to be a weird shape to fit the dimple or just get mashed in. A countersunk substructure would not fit the mating dimpled skin either. I would like to see the reference to this rule, but don't put much weight in it based on what I have seen.
Mike
-
Russ,
Sorry my reply has taken so long. I had made up the page that will be linked below, but didn't want to switch this to a dimple die thread. Since there is a dimple die thread going on, I decided that I would post there. I do still intend on getting a tutorial on how to 'see' the difference in results of dies and techniques.
<snip from: http://www.vansairforce.com/community/showthread.php?t=81949&page=2>
At Cleaveland we perfected the 1942 springback dimple die design. The term springback refers to the angle of the 'faces' of the dies rather than the angle of the 'funnel'. The male die has a concave face of a couple degrees where the female has a convex face. These angles as well as some of the other diameters and angles are critical as to how they fit the rivet, and how flat the material ends up after dimpling. Most of our competitors now have attempted to copy our 1992 die geometry, some with more success than others, but they still don't measure up to what a Cleaveland Dimple Die can do. To read more about exactly how our dies were perfected and how we keep them that way you can read the following page that I have put together to answer this question.
http://www.cleavelandtoolstore.com/custompages/springback-dimple-dies.html
As some builders don't know what to look for I will create a short tutorial on how to evaluate the dimpled skin and post it here shortly.
The statement about the tank dies for the substructure is certainly correct. The tank dies are about .007" deeper than standard to account for proseal under the rivet head. They work better for nesting than two dimples from the same die. Most builders don't want to buy another set for the substructure, but for those discerning builders we do carry a "substructure" die (DIE4263SS) that is .011" deeper so that the dimples will nest perfectly.
-
Mike Lauritsen - Cleaveland Aircraft Tool
A theory question for you Mike. I read your post on Springback Dimple Die history... very interesting.
One thing I wondered was how close to cylindrical/vertical the holes end up being. I saw a reference from a manufacturer somewhere that says the cylinder of the hole after dimpling should be vertical, with no edges pointing in.
Here's an example of what I imagine they don't want to see:
from: Vans Riveting Specs
In your Springback Dimple Die history, there's a quote from a book stating:
"The cylindrical hole was obtained by drilling the initial hole in the sheet to a diameter slightly smaller than that of the mandrel on the male die (which served to center the die in the hole, as in fig. 3c); the suitably tapered mandrel then forged the hole to the desired shape in the dimpling operation. The hole could not be made too small, however, lest radial cracks appear around its edge."
Kitbuilders typically match drill their holes first, then dimple them. The holes are a bit larger than the dimple mandrel of the die. So wouldn't this cause the non vertical (conical) hole sides as shown in the figure above? Or are the die mandrels actually slightly smaller than the hole when dimpling so that they are formed vertically?
Hope this all makes sense.
Thanks!
Russ
Nice question Russ! After 20 years I don't hear new questions very often unless it is in reference to a new kit or new tool...
First off the cylindrical hole is important, that is why we have been suggesting #40 and #30 reamers to enlarge the prepunched holes since the first prepunched kits. The reamer with it's 45deg. entry bevel was designed to start itself perfectly centered in a hole and enlarge it to a perfectly sized and perfectly cylindrical hole. Machinists have been doing this 'forever' and many other industries know that the only way to get a good fit and finish is to ream a hole. With the matched hole prepunch the reamer takes the same time as a drill with better results... why wouldn't you ream them? There is less burr because with the drill bit 'chattering' because of the lack of center to follow causes 'tearout' and triangular shaped holes. This tearout makes deburring a challenge and often leads to over deburring. Over deburring by just 0.0025" on each side of a 0.025" skin takes 20% of your skin thickness away!! But all that is old news (question answered many times before).
The interesting part is the tapered pilot die. We decided against that idea right away, and here is why. If you are using a good quality tool to align the dies (like the C-frame or squeezer) the male and female die will be lined up with each other. (This is why a squeezer that has the ram going through the yoke is essential, when the yoke has pressure applied, it will rock back, the ram MUST go with it or alignment is not only lost, but due to the side load now induced the whole process gets out of control). If alignment is maintained, the male will be concentric with the female, the straight shank die will slip through the hole and the funnel of the die will center the cylindrical hole on the cylindrical die funnel. This lines the hole up with the die. If the male pilot was slightly tapered the side load forces on the sheet would climb quickly and we were afraid of what that might mean for the hardness of the Aluminum at that critical point for cracking. As the dimple is made the hole expands anyway so any tight tolerance in the pilot/hole interface is lost. We did do extensive testing to look at the behavior of the Aluminum sheet of various thickness under a microscope when smaller diameter holes were used with a smaller diameter pilot ( pay attention #41 guys
). Our goal was to make a die and hole combination (this is prior to prepunched kits) that would result in a tight fit with a 3/32" rivet. Radial stress cracks occurred quickly when reducing the pilot size. When we started with screw dies we again did similar testing so that standard clekos could be used for #6 and #8 screw holes. Again we couldn't get it done because of the stress cracks induced. We finally determined that with millions of hours behind the tried and true #40, #30 starting hole, safety is on the side of keeping it there. After learning about strength of materials in engineering classes I am quite happy that we did not find a way to make the un-expanded 3/32" rivet work. I have never done the math but I am guessing that is a fairly significant reduction in area.As for the last question about the sides of the cylindrical hole being vertical... I have never heard reference to that and doubt it's origins. I can see how that could be done, but I don't see much advantage other than the shop head having a bit flatter surface to sit against. This would also make the rivet need to be a weird shape to fit the dimple or just get mashed in. A countersunk substructure would not fit the mating dimpled skin either. I would like to see the reference to this rule, but don't put much weight in it based on what I have seen.
Mike
-
